Interesting plot line, albeit a little random. I'll give you a little advice on how to improve:

- In the beginning scene, where the main character is walking, I see you just animated the legs and arms and had the body move in a straight line. If you watch any cartoon or real person, their bodies move up and down as they're walking.

- With sites like freesound.org or even YouTube, there's no reason why you can't find some decent sound effects. Don't try to make them with your mouth; it really takes away from the video and makes people rate it lower.

- If you intend on taking this story further, I'd recommend actually coming up with a plot ahead of time rather than just improvising it on the spot. Storyboard it out, and then you can foreshadow future plot elements and make the whole thing flow better. Give your characters a bit of background that you can draw on for future episodes or to determine how they'll act in certain situations. I'd recommend watching https://youtu.be/FoH82EuXfjg for advice on how to do that.

- Also, try to avoid falling into that stereotypical anime "escalation" trap where the main character always has to save the world. That's discussed really well in this video https://youtu.be/kLIbI1FI6N4 if you have time to sit through it. Try to avoid just recycling plot elements from other cartoons and anime.

- Other than that, you just need to work on adding more detail to the backgrounds and improving your drawings. A lot of that comes with time and practice; nobody starts out perfect. If you keep working at it, you'll improve eventually. Good luck, and most importantly, have fun!